As a technique for reducing a variation in an air-fuel ratio between cylinders of an internal combustion engine, for example, as described in JP-A 2013-253593, has been proposed a technique for estimating an air-fuel ratio of an individual cylinder on the basis of a sensed value of an air-fuel ratio sensor set in an exhaust gas collection part of the internal combustion engine and for performing an individual cylinder air-fuel ratio control of controlling the air-fuel ratio of the individual cylinder on the basis of an estimated air-fuel ratio of the individual cylinder. Further, the technique determines whether or not an air-fuel ratio sensing timing is shifted from a proper air-fuel ratio sensing timing on the basis of the estimated air-fuel ratio while performing the individual cylinder air-fuel ratio control. When the technique determines that the air-fuel ratio sensing timing is shifted from the proper air-fuel ratio sensing timing, the technique makes an air-fuel ratio sensing timing correction of correcting the air-fuel ratio sensing timing.
Further, considering that a time lag from the time when an exhaust gas exhausted from the individual cylinder reaches near the air-fuel ratio sensor to the time when the air-fuel ratio is sensed by the air-fuel sensor (that is, a response delay in an exhaust system) is changed due to the load and the rotation speed of the internal combustion engine, the technique sets an air-fuel ratio sensing timing (sampling timing of the output of the air-fuel ratio sensor) of the individual cylinder according to the load and the rotation speed of the internal combustion engine.
Still further, for the purpose of enhancing an exhaust gas cleaning rate of a catalyst for cleaning an exhaust gas of the internal combustion engine, the technique performs a main feedback control of calculating a main feedback correction value in such a way that an air-fuel ratio of the exhaust gas matches a target air-fuel ratio on the basis of the output of the air-fuel ratio sensor set in an exhaust pipe and of evenly correcting the air-fuel ratio of the individual cylinder (for example, fuel injection amount).
When a misfire in the internal combustion engine (for example, a consecutive misfire in a specific cylinder) is caused while the individual cylinder air-fuel ratio control is performed, the estimation accuracy of the air-fuel ratio of the individual cylinder can be impaired due to the effect of the misfire in some cases. In this case, an estimated air-fuel ratio of the individual cylinder can diverge and hence can make it difficult to correctly control the air-fuel ratio of the individual cylinder (to control the air-fuel ratio of the individual cylinder in such a way that a variation in the air-fuel ratio between the cylinders becomes small), which hence can cause an exhaust emission to deteriorate.
Further, when the internal combustion engine is rapidly accelerated and then rapidly decelerated, an exhaust flow rate can be greatly decreased and hence the flow of the exhaust gas can stagnate in some cases. When the flow of the exhaust gas near the air-fuel ratio sensor stagnates while the individual cylinder air-fuel ratio control is performed, in some case, the amplitude of the output of the air-fuel ratio sensor can be reduced due to the stagnation of the exhaust gas, which can make it difficult to correctly read the output of the air-fuel ratio sensor at each air-fuel ratio sensing timing of the individual cylinder. In this case, an estimated cylinder mistake (a mistake of correspondence relationship between an individual cylinder and an estimated air-fuel ratio) can be caused to thereby make it difficult to correctly control the air-fuel ratio of the individual cylinder, which hence can cause the exhaust emission to deteriorate. When the air-fuel ratio sensing timing is corrected in this state, there is also a possibility that the estimation accuracy of the air-fuel ratio of the individual cylinder will further deteriorate.
An exhaust flow rate can be changed by an operation condition (for example, an ignition timing) other than the load and the rotation speed of the internal combustion engine, and when the exhaust flow rate is changed, the response delay of the exhaust system is changed along with the change in the exhaust flow rate and hence the phase of an output waveform of the air-fuel ratio sensor is changed. Hence, a proper air-fuel ratio sensing timing of the individual cylinder is also changed.
For this reason, even if the air-fuel ratio sensing timing of the individual cylinder is set according to the load and the rotation speed of the internal combustion engine at the time of performing the individual cylinder air-fuel ratio control, if the exhaust flow rate is changed by the operation condition other than the load and the rotation speed of the internal combustion engine, the air-fuel ratio sensing timing of the individual cylinder can be shifted from the appropriate air-fuel ratio sensing timing of the individual cylinder. When the air-fuel ratio sensing timing of the individual cylinder is shifted from the appropriate air-fuel ratio sensing timing of the individual cylinder, the estimation accuracy of the air-fuel ratio of the individual cylinder is reduced, which hence makes it difficult to correctly control the air-fuel ratio of the individual cylinder (to control the air-fuel ratio of the individual cylinder in such a way that the variation in the air-fuel ratio between the cylinders becomes small).
When the variation in the air-fuel ratio between the cylinders of the internal combustion engine is comparatively large and the load of the internal combustion engine is large (that is, the exhaust flow rate is large), the amplitude of the output of the air-fuel ratio sensor becomes large and hence the output of the air-fuel ratio sensor is greatly changed. In this case, a main feedback correction value by the main feedback control is greatly changed, which hence can cause the variation in the air-fuel ratio between the cylinders to increase. For this reason, it is difficult to reduce the variation in the air-fuel ratio between the cylinders (to make an estimated air-fuel ratio of the individual cylinder converge) by the individual cylinder air-fuel ratio control, which hence can cause the exhaust emission to deteriorate.